How does the tool know which direction I want?

If your input contains a slash it is treated as a MIME type (for example application/json) and the tool returns the associated extensions. Otherwise it is treated as a file extension — with or without a leading dot — and the tool returns the matching MIME type.

Is the lookup done online?

No. The registry is bundled into the page, so every lookup is instant and works offline. Nothing is sent over the network and no input is logged.

Which MIME type should I use for JavaScript?

For files served to browsers, text/javascript is now the standard registered type for .js and .mjs, though application/javascript still works and is widely seen. The tool notes this where relevant.

Why does serving the right Content-Type matter?

Browsers may MIME-sniff a response and, for security, refuse to run a script or apply a stylesheet that is served with the wrong type. Sending the correct Content-Type header prevents broken pages and blocks a class of content-confusion attacks.

What if my extension is not in the registry?

The tool covers the common web, document, image, audio, video, font and archive types. For an unknown binary file, application/octet-stream is the safe default — it tells the browser to download rather than try to render the file.

What is the MIME Type Lookup & File Extension Map?

Free in-browser MIME type lookup. Enter a file extension to find its MIME type, or a MIME type to find its associated extensions, using a built-in registry. Bidirectional, instant and offline — no network request and nothing uploaded. It runs free in your browser on Gera Tools, with nothing uploaded.

MIME Type Lookup & File Extension Map

Name: MIME Type Lookup & File Extension Map
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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The MIME Type Lookup & File Extension Map answers both questions developers ask about content types: “what MIME type does this extension use?” and “what extensions does this MIME type cover?” It searches a built-in registry instantly and entirely offline.

How it works

The tool ships with a curated subset of the IANA / mime-db registry mapping each MIME type to its associated file extensions. When you type:

an input containing a slash (e.g. application/pdf) it is treated as a MIME type and the tool lists every registered extension;
otherwise the input is treated as a file extension (a leading dot is optional) and the tool returns the matching MIME type along with that type’s full extension list.

If an exact match is not found, the tool falls back to a substring search so a partial MIME fragment still surfaces likely candidates.

Worked examples

You type	Direction	Result
`png`	extension → MIME	`image/png`
`image/jpeg`	MIME → extensions	`.jpg`, `.jpeg`, `.jpe`
`pdf`	extension → MIME	`application/pdf`
`application/json`	MIME → extensions	`.json`
`wasm`	extension → MIME	`application/wasm`
`xyz` (unknown)	extension → MIME	`application/octet-stream` suggested

Why the Content-Type header matters

When a browser fetches a resource, it uses the Content-Type response header to decide how to handle it. A CSS file served as text/plain will not be applied as a stylesheet. A script served as image/jpeg will not execute. Modern browsers implement strict MIME checking — deliberately — as a security measure against content-confusion attacks where a malicious server disguises executable content as something harmless.

Getting the type right also matters for:

Download behavior. A Content-Disposition: attachment header pairs with a meaningful MIME type so operating systems know which application to open the file with.
Compression decisions. Proxies and CDNs often decide whether to compress a response based on MIME type, so serving JavaScript as text/javascript rather than text/plain may activate gzip or brotli compression.
Security headers. X-Content-Type-Options: nosniff tells browsers not to sniff a type that conflicts with the declared one — you must declare the right type for that header to work without breaking things.

Common types to look up

Web fonts — .woff2 → font/woff2; .woff → font/woff
Modern images — .webp → image/webp; .avif → image/avif
Media — .mp4 → video/mp4; .webm → video/webm
Data — .jsonld → application/ld+json; .ndjson → application/x-ndjson
Manifests — .webmanifest → application/manifest+json

For unknown binary formats, application/octet-stream remains the safe catch-all: it prompts a download rather than inline rendering, which is safer than guessing wrong.